The present invention relates to communication systems employing a power-limited repeater, and more particularly to a satellite communications system wherein the communication links are subject to rapid onset signal attenuation.
Extensive use is currently being made of satellite repeaters in a wide variety of communications applications. The number and size of the Earth-based stations which may simultaneously participate in the use of any given satellite repeater is constrained by the limited availability of power at the satellite. To optimize use of the satellite, then, it is necessary that the power which is available be allocated as efficiently as possible.
Unfortunately, these satellite communications links are subject to changing levels of signal attenuation due, for example, to the occurrence of rainfall between the satellite and a communicating terminal. In order to prevent loss of signal at the receiving station due to this attenuation, the amount of satellite transponder power employed in each individual communication link must be tailored to include some degree of excess power, known as "margin". The use of large amounts of satellite power in providing high margin on each communication link represents a very inefficient use of satellite transponder power however, since the majority of the time these additional measures of transponder power will not be necessary to insure adequate signal quality at the receiving station. If some more efficient manner of allocating satellite power were employed, the satellite could be used more effectively to provide, e.g., communication between a greater number of terminals, higher data rates on the existing links, communications between more widely-spaced terminals, etc.
An adaptive power control scheme could provide the required efficient allocation of available satellite power. The implementation of such a scheme is simplified to some extent by the fact that the power output of most satellites is a function of the power received thereby. Thus, the power output of the satellite may be adjusted by the simple expediant of varying the power which is transmitted to it.
This has been recognized and studies have been made of the possibility of allocating only enough margin to each communications link as is necessary to maintain signal quality at a desired level along that link. (See, for example "A Statistical Analysis of Transmit Power Control to Compensate Up- and Down-Link Fading in an FDMA Satellite Communications System", Lyons, Robert G., IEEE Trans. on Comm., Vol. Comm-24, No. 6, June 1976, and references cited therein.) These studies contemplate the use of statistical measurement of received signal strength as a basis for determining the power to be assigned to any given communications link. Since this measurement was to be accomplished on a statistical, long term basis, short-term variations in attenuation along a link could not be taken into account. These previously proposed systems would therefore still require that sufficient margin be assigned to each link to overcome these short-term variations.
These short-term variations may be quite large. It is possible, for example, for the sudden onset of a rainstorm in a communications link to introduce a rapid onset attenuation on the order of0.1 db per second, or more. The margins necessary to overcome attenuation variations of this magnitude are correspondingly large. Each satellite communication link must therefore either tolerate short-term outages due to this rapid onset attenuation, or must be assigned a significant amount of margin in order to overcome the fades in signal strength resulting therefrom.
These studies at power control in satellite communications systems have further contemplated the inclusion of sophisticated and expensive equipment at each terminal for performing the power control function. Furthermore, those studies suggesting the use adaptive of power control have heretofore required the inclusion of additional communication channel capacity for communicating power control information between stations. These can be a significant factors in determining whether or not power control is economically feasible in a satellite communications system. It would clearly be desirable to provide a system wherein little additional equipment were required, little or no modification to existing equipment were required, and little additional communication channel capacity were necessary.